The term power transistor refers to all those types of transistors which at a high current and a high voltage emit a high power on the collector side.
These requirements are coupled with several conditions which cannot be readily met by the usual planar technological methods used to manufacture power transistors.
A first condition to be met during the manufacture of a power transistor is a high backward voltage collector-base junction. A high backward voltage collector-base junction necessitates a flat doping profile of the base zone. When the usual methods are applied a flat doping profile of the base is obtained by producing the base in a long-term diffusion step. This long-term diffusion leads to a relatively deep base zone. However, the frequency limit of a transistor is essentially a function of the base width and the capacity of the collector depletion layer. In order to achieve a base width which is small with regard to the high-frequency characteristic, it is essential to have a deep emitter zone because of the deep base zone necessary to ensure a flat doping profile. This second condition, however, runs contrary to a third condition a power transistor structure is expected to meet. This third condition arises from the necessity to have a high emitter efficiency. A high emitter efficiency, as is known, can be achieved by selecting a large area emitter zone in connection with a high doping and a steep emitter doping profile, respectively.
A large area emitter zone produces the so-called current density effect. In a transistor with a circular emitter and a base terminal surrounding the latter in the form of a circular ring the current paths extend from the base contact below the emitter to the emitter depletion layer. As the base zone has a relatively low doping it has a relatively high transverse resistance. This, in turn, leads to a voltage drop from the emitter periphery towards the center of the emitter area and thus from the periphery in a radial direction to a decrease in the forward voltage of the emitter depletion layer. At higher currents this effect becomes noticeable in that the injection decreases substantially in the direction of the center of the emitter area, thus limiting the emitter current. At the current densities desirable for power transistors and at high voltages the current density leads to a current or a voltage limitation. A solution of this problem is known from German Pat. No. 1,281,036, for example, according to which the emitter zone is screen-shaped, so that parts of the base zone can engage the apertures of the screen-shaped emitter zone. While eliminating the current density effect this ensures an increase in the emitter circumference/emitter area ratio and thus a relatively high emitter current density.
However, the further problem of having a highly doped emitter zone in planar technology close to the collector-base junction in order to reduce the base width to a minimum is extremely difficult to solve where a deep base diffusion is necessary to ensure a flat base profile. The reason for this is that the great depth of the collector-base junction necessitates a very long emitter diffusion step, which considerably reduces the efficiency of the emitter. Apart from this, it is necessary in most cases to provide for very thick masking oxide layers or other masking layers as emitter diffusion masks.